1. Field of the Invention
This invention relates to a wavelength division multiplexed optical fiber transmission equipment. More particularly, it relates to a wavelength division multiplexed optical fiber transmission equipment which is capable of increasing the transmission capacity of an optical fiber transmission system by suppressing interference between codes caused by four-wave mixing.
2. Description of the Related Art
Since an optical fiber transmission system which makes use of a wavelength division multiplexed signal (to be abbreviated as WDM signal hereinafter) can increase the transmission capacity of a transmission line without making modifications on the transmission channel, its technology is expected to be applied in a future optical fiber transmission system for trunk lines. When a WDM signal is used, if four-wave mixing between signal wavelengths is existent, four-wave mixing generated from two signal lights adjacent to a signal light causes an inter-symbol interference in the case of equal spacings between signal wavelengths. Thereby, the characteristics of the system such as the maximum transmission distance and the maximum number of channels of the WDM signal are limited. As a technology for preventing deterioration in characteristics caused by this four-wave mixing, there is known one in which the spacing between arbitrary two wavelengths is made unequal (F. Forghieri et al., IEEE Photon. Technol. Lett. vol. 6, no. 6, pp. 754-756).
An example of the technology is described with reference to FIG. 10. FIG. 10 shows the wavelengths of signal lights allocated to respective eight channels as the axis of abscissas. As evident from the figure, the channel spacing between first and second channels is 0.8 nm, the channel spacing between second and third channels is 0.9 nm, the channel spacing between third and fourth channels is 1.2 nm, and so on. In this way, the spacing between arbitrary two chanells is made unequal to the spacing between any other two channels.
By making the spacings unequal, it is possible to effectively suppress an inter-symbol interference caused by four-wave mixing generated by signal lights adjacent to another signal light.
However, when the spacings between respective two wavelengths are made unequal, a required bandwidth of an optical signal becomes wider than when the spacings between respective two wavelengths are equal. As the result, the number of wavelengths able to be multiplexed is reduced. The reason for this is as follows. Since the minimum channel spacing of a WDM signal is limited by the performance of an optical de-multiplexer used for the separation of the WDM signal as a matter of course, when x represents the minimum channel spacing, a bandwidth nx is sufficient for a system in which an n number of wavelengths are multiplexed with equal spacings while a bandwidth for multiplexing an n number of wavelengths is much larger than nx in a system with unequal spacings in which the spacings between respective two wavelengths must be set larger than x.
In an example of the system shown in FIG. 10, since the minimum channel spacing x is 0.7 nm, when the spacings between respective two wavelengths are made equal, the bandwidth of optical signals for channels 1 to 8 is 4.9 nm. In contrast to this, when the spacings between arbitrary two wavelengths are made unequal, the bandwidth needs to be 7 nm, about 1.4 times wider than when the spacings between respective two wavelengths are made equal as understood from the figure.
Apart from a technology for making unequal the spacings between signal wavelengths, there is known a technology for intentionally arranging adjacent optical signals such that the states of polarizations of these signals cross each other, making use of the fact that the generation efficiency of four-wave mixing is high when the states of polarizations of optical signals associated with the generation of four-wave mixing match with each other and low when the states cross each other.
However, this technology is unsatisfactory as a technology for suppressing generation of four-wave mixing when it is applied in a long-distance wavelength division multiplexed transmission system because the states of polarizations of adjacent signals cannot be maintained due to birefringence of an optical fiber after the signals are transmitted through some distance. Furthermore, when the states of polarizations of adjacent optical signals are caused to cross each other, the states of polarizations of signal wavelengths with one wavelength interposed therebetween match each other. As the result, the generation efficiency of four-wave mixing is increased.